We have investigated the effect of low oxygen partial pressure (pO2 of 0.05%) on the microstructure and magnetic properties of NdBa2Cu3Oy, EuBa2Cu3Oy, GdBa2Cu3Oy, and (Nd, Eu, Gd)Ba2Cu3Oy. The heat treatment profiles for the melt processes were determined according to the peritectic decomposition temperature obtained by thermal analysis. All the samples showed relatively high Tc in the range of 93.6-96.1 K. Optical and scanning electron microscopy showed that in the Gd-123 matrix the secondary phase particles formed sub-micron-sized bundles. The scaling study of the normalized volume pinning forces showed that ΔTc type pinning is dominant in all the samples. The Gd-123 sample exhibits the highest zero-field critical current density of 88 000 A cm-2 at 77 K for the H ∥c-axis, while the highest secondary peak Jc of 65 000 A cm-2 was achieved in Nd123 at 1 T and 77 K for the H ∥c-axis.
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